1. Technical Field
This invention relates to printers printing labels and tags on media and, more particularly, to sensing apparatus for determining edge-indicating changes in thickness of a moving web from a fixed surface over which the web is moving comprising, a sensor disposed above the fixed surface measuring the distance to a surface passing beneath it and outputting a first signal reflecting the distance; an amplifier amplifying the first signal to a usable level second signal; and, a detector sensing changes in the second signal beyond a pre-established threshold amount and outputting a third signal indicating an edge has been found.
2. Background Art
When printing tags or labels removably and adhesively carried by a strip backing, there is a need for the printer to sense the longitudinal position of the tags or labels in order to print on them in proper alignment and subsequently cut between them if the printer includes a cutter mechanism.
The environment is depicted in FIG. 1 and several prior art approaches to solving the problem are shown in FIGS. 2 through 4 in conjunction therewith. The tags/labels 10 are attached to and carried by a strip of backing 12. Typically, there are in the order of 500 or 1,000 tags/labels 10 carried by the strip of backing 12.
In FIG. 2, a light beam 14 from a sending unit 16 is sensed by a receiving unit 18 to produce a signal on line 20 which can be sensed. The receiving unit 18 senses the intensity of the light beam 14 striking it after passing through the tags/labels 10 and strip of backing 12. The signal on the line 20 is supposed to be proportional to the intensity and thereby the amount of material through which the light beam 14 passed. In theory, where the intensity is high, the light beam 14 has not passed through a tag/label 10 so the gap between adjacent ones must be in the path of the light beam 14.
In FIG. 3, the backing 12 contains a notch 22 (or hole) at the location of the gap between adjacent tags/labels 10. The same sending unit 16 sensed by receiving unit 18 again produces a signal on line 20. In this case, however, it is sensing the presence or absence of the backing 12.
In FIG. 4, the light beam 14 from the sending unit 16 to the receiving unit 18 is reflected from the backing 12. A dark or reflective spot 24 is placed at the gap. An increase or decrease in the reflected light beam 14 (depending on whether less reflective or more reflective than the surface of the backing 12) determines the presence of the spot 24 and, therefore, the gap.
The foregoing prior art approaches have proven to be less than desirable due to several factors. The first is the wide range of possible opacity ranges encountered in differing medias. Both the overall opacity and the relative delta opacities can vary widely. Setting light source intensity and amplification gain values that will work for all media is near to impossible. Second, varying ambient light conditions can adversely affect both transmissive and reflective sensor accuracy. Finally, the optical components found in (relatively) low cost sensor pairs vary widely in their light output (IR LEDs) and gain (photo-transistors). These factors must be compensated for in circuit designs and software detection algorithms. These components also exhibit a tendency to drift with age or vary with ambient temperature changes making calibration even more difficult.
Wherefore, it is an object of this invention to provide methods a associated apparatus which will allow gaps between adjacent media items carried by a backing strip to be sensed simply and reliably.
It is an object of this invention to provide methods a associated apparatus which will allow gaps between adjacent media items carried by a backing strip to be sensed in a manner which is not dependent on a constant and repeatable opacity or reflectivity of the measured components.
Other objects and benefits of this invention will become apparent from the description which follows hereinafter when read in conjunction with the drawing figures which accompany it.